Student or Learner
1. I have not done the homework either.
2. I have not done the homework neither.
3. Neither Sam or Mary has done the homework.
4. Either Sam or Mary has done the homework. (I want to show that both of them have the homework done.)
Above are the contexts of my own, I would like to know if they are grammatical and natural.
1 is correct. 3 would be OK with "nor" instead of "or". 2 and 4 are incorrect.
Is it correct that "neither...nor" means both of the subjects have not done the action, while "either...or" means only one subject has done the action?
"Either Sam or Mary should be able to help you" can mean that both could help, but you only need one, so you can choose either Sam or Mary. Or it could mean that only one [or at least one] will be able to help.